I encountered an issue building BTF kernels for 32-bit armhf, where many
functions are missing in BTF data:
LD vmlinux
BTFIDS vmlinux
WARN: resolve_btfids: unresolved symbol vfs_truncate
WARN: resolve_btfids: unresolved symbol vfs_fallocate
WARN: resolve_btfids: unresolved symbol scx_bpf_select_cpu_dfl
WARN: resolve_btfids: unresolved symbol scx_bpf_pick_idle_cpu_node
WARN: resolve_btfids: unresolved symbol scx_bpf_pick_idle_cpu
WARN: resolve_btfids: unresolved symbol scx_bpf_pick_any_cpu_node
WARN: resolve_btfids: unresolved symbol scx_bpf_pick_any_cpu
WARN: resolve_btfids: unresolved symbol scx_bpf_kick_cpu
WARN: resolve_btfids: unresolved symbol scx_bpf_exit_bstr
WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_nr_queued
WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_move_vtime
WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_move_to_local
WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_move
WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_insert_vtime
WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_insert
WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_vtime_from_dsq
WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_vtime
WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_from_dsq_set_vtime
WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_from_dsq_set_slice
WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_from_dsq
WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch
WARN: resolve_btfids: unresolved symbol scx_bpf_destroy_dsq
WARN: resolve_btfids: unresolved symbol scx_bpf_create_dsq
WARN: resolve_btfids: unresolved symbol scx_bpf_consume
WARN: resolve_btfids: unresolved symbol bpf_throw
WARN: resolve_btfids: unresolved symbol bpf_sock_ops_enable_tx_tstamp
WARN: resolve_btfids: unresolved symbol bpf_percpu_obj_new_impl
WARN: resolve_btfids: unresolved symbol bpf_obj_new_impl
WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
WARN: resolve_btfids: unresolved symbol bpf_iter_task_vma_new
WARN: resolve_btfids: unresolved symbol bpf_iter_scx_dsq_new
WARN: resolve_btfids: unresolved symbol bpf_get_kmem_cache
WARN: resolve_btfids: unresolved symbol bpf_dynptr_from_xdp
WARN: resolve_btfids: unresolved symbol bpf_dynptr_from_skb
WARN: resolve_btfids: unresolved symbol bpf_cgroup_from_id
NM System.map
After further debugging this can be reproduced more simply:
$ pahole -J -j --btf_features=decl_tag,consistent_func,decl_tag_kfuncs .tmp_vmlinux_armhf
btf_encoder__tag_kfunc: failed to find kfunc 'scx_bpf_select_cpu_dfl' in BTF
btf_encoder__tag_kfuncs: failed to tag kfunc 'scx_bpf_select_cpu_dfl'
$ pfunct -Fbtf -E -f scx_bpf_select_cpu_dfl .tmp_vmlinux_armhf
<nothing>
$ pfunct -Fdwarf -E -f scx_bpf_select_cpu_dfl .tmp_vmlinux_armhf
s32 scx_bpf_select_cpu_dfl(struct task_struct * p, s32 prev_cpu, u64 wake_flags, bool * is_idle);
$ pahole -J -j --btf_features=decl_tag,decl_tag_kfuncs .tmp_vmlinux_armhf
$ pfunct -Fbtf -E -f scx_bpf_select_cpu_dfl .tmp_vmlinux_armhf
bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct * p, s32 prev_cpu, u64 wake_flags, bool * is_idle);
The key things to note are the pahole 'consistent_func' feature and the u64
'wake_flags' parameter vs. arm 32-bit registers. These point to existing
code handling arguments larger than register-size, allowing them to be
BTF encoded but only if structs.
Generalize the code for any argument type larger than register size (i.e.
size > cu->addr_size). This should work for integral or aggregate types,
and also avoids a bug in the current code where a register-sized struct
could be mistaken for larger. Note that zero-sized arguments will still
be marked as inconsistent and not encoded.
Fixes: a53c58158b76 ("dwarf_loader: Mark functions that do not use expected registers for params")
Tested-by: Alexis Lothoré <alexis.lothore@xxxxxxxxxxx>
Tested-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
Signed-off-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
---
v2 -> v3:
- Added Tested-by: from Alexis and Alan.
- Revert support for encoding 0-sized structs (as v1) after discussion:
https://lore.kernel.org/dwarves/9a41b21f-c0ae-4298-bf95-09d0cdc3f3ab@xxxxxxxxxx/
- Inline param__is_wide() and clarify some naming/wording.
v1 -> v2:
- Update to preserve existing behaviour where zero-sized struct params
still permit the function to be encoded, as noted by Alan.
---
dwarf_loader.c | 37 ++++++++++++-------------------------
1 file changed, 12 insertions(+), 25 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index e1ba7bc..134a76b 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -2914,23 +2914,9 @@ out:
return 0;
}
-static bool param__is_struct(struct cu *cu, struct tag *tag)
+static inline bool param__is_wide(struct cu *cu, struct tag *tag)
{
- struct tag *type = cu__type(cu, tag->type);
-
- if (!type)
- return false;
-
- switch (type->tag) {
- case DW_TAG_structure_type:
- return true;
- case DW_TAG_const_type:
- case DW_TAG_typedef:
- /* handle "typedef struct", const parameter */
- return param__is_struct(cu, type);
- default:
- return false;
- }
+ return tag__size(tag, cu) > cu->addr_size;
}
static int cu__resolve_func_ret_types_optimized(struct cu *cu)
@@ -2942,9 +2928,9 @@ static int cu__resolve_func_ret_types_optimized(struct cu *cu)
struct tag *tag = pt->entries[i];
struct parameter *pos;
struct function *fn = tag__function(tag);
- bool has_unexpected_reg = false, has_struct_param = false;
+ bool has_unexpected_reg = false, has_wide_param = false;
- /* mark function as optimized if parameter is, or
+ /* Mark function as optimized if parameter is, or
* if parameter does not have a location; at this
* point location presence has been marked in
* abstract origins for cases where a parameter
@@ -2953,10 +2939,11 @@ static int cu__resolve_func_ret_types_optimized(struct cu *cu)
*
* Also mark functions which, due to optimization,
* use an unexpected register for a parameter.
- * Exception is functions which have a struct
- * as a parameter, as multiple registers may
- * be used to represent it, throwing off register
- * to parameter mapping.
+ * Exception is functions with a wide parameter,
+ * as single register won't be used to represent
+ * it, throwing off register to parameter mapping.
+ * Examples include large structs or 64-bit types
+ * on a 32-bit arch.
*/
ftype__for_each_parameter(&fn->proto, pos) {
if (pos->optimized || !pos->has_loc)
@@ -2967,11 +2954,11 @@ static int cu__resolve_func_ret_types_optimized(struct cu *cu)
}
if (has_unexpected_reg) {
ftype__for_each_parameter(&fn->proto, pos) {
- has_struct_param = param__is_struct(cu, &pos->tag);
- if (has_struct_param)
+ has_wide_param = param__is_wide(cu, &pos->tag);
+ if (has_wide_param)
break;
}
- if (!has_struct_param)
+ if (!has_wide_param)
fn->proto.unexpected_reg = 1;
}
--
2.34.1
